October 13, 2010

Ceramic Bearings. Does Size or Quantity Matter?

I want to shed some light on ceramic bearings. Now that they have been out for a good 2+ years, I'm just now seeing more of them than ever and a disturbing number of them that I've seen already have pitted races. I'm going to write more on this sometime in the future, but for now, I want to show you something that came thru my shop recently. The first photo is the bearing from a SRAM Blackbox GXP Ceramic bottom bracket:

The next photo is of a Shimano 7800 bottom bracket with Enduro Zero Ceramic bearings:

Now, everything else being equal, in this application, is it better to have fewer but larger diameter ceramic ball bearings or more but smaller ceramic ball bearings? Any bearing experts out there? You thoughts please. Thanks for checking in. -John

UPDATE:

Here's some interesting info that I received from JF via e-mail...

It seems that there is a magic exponent here, which is 2/3:

1. The maximum stress on a single ball decreases roughly with the 2/3power of the ball radius.

2. The maximum load capacity grows with the number of balls to the 2/3power as well, or so they say here:http://machinedesign.com/article/radial-bearings-1115

This means that all exponents cancel out, and the 11 ball Shimanobearing will have the same load capacity as the 16 ball SRAM if itsballs are 16/11 = 1.45, so 45% larger in diameter. It's hard to tellfrom the pictures, but if the inside diameter of both bearings is thesame, then the photos show something around a 20% larger diameteronly. That would mean that, all other things being equal, SRAM wouldhave somewhere around 13% more load capacity.

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